Use SVG badges instead of PNG badges

| before | after|
| --- | --- |
| ![](https://travis-ci.org/cainus/node-coveralls.png?branch=master) | ![](https://travis-ci.org/cainus/node-coveralls.svg?branch=master) |
| ![](https://coveralls.io/repos/cainus/node-coveralls/badge.png?branch=master) | ![](https://img.shields.io/coveralls/cainus/node-coveralls.svg?branch=master) | 

SVG badges look beautiful on retina displays.